I use an inverter regularly in my car to power a laptop for gps and run my mp3 player, i bought a 300watt one with two outlets, though 300 watts can only be pulled from the direct connection to the battery.
I've been hoping to setup a decent sound system initially with a large deep cycle marine battery for outdoor funk parties, though I haven't really made the step to buy speakers yet, but essentially you wouldn't want to run it from your cigarette lighter, even a decent size tv or a blender I would run from the batt itself.
the nice thing about the inverter I have is an LED readout of the volts my batt has, nice to see exactly where its charged, last night I was installing a new stereo and had the dome light on the whole time, v's dropped to 10.5 when they are usually after my commute around 14.1, kinda nice to know how much juice I have for the cold morning starts here in boston.
here's a question: Put an oversized marine battery as you starter, it would charge via the alternator, I have seen some cars using marine batts as starters.
